1. Field of the Invention
The invention relates to a free channel detecting method and device, and more particularly to an improved free channel detecting method and device which can detect a free channel without fail in a communication device which performs time division multiplex communication on a plurality of channels by a modulation method for linear modulation.
2. Description of the Related Art
Conventional communication devices for communicating on a plurality of channels are known as portable radiotelephones and the like.
Such a portable radiotelephone searches a plurality of radio channels for a free radio channel and uses it for communication.
To search for this free radio channel, it is configured that for example, envelope detection of a received signal is performed, a mean value of envelope is determined, and it is determined whether a channel to be used is free or not according to the mean value.
The above-described configuration of determining the mean value does not have any disadvantage for a communication method using a continuous signal, but when a time division channel is used for communication, it has a disadvantage in the response characteristic of a mean circuit for determining the mean value.
Since the mean circuit for determining the mean value is generally formed of a resistor and a capacitor in combination, communication through the time division channel which has a received signal in a burst state takes a long time before the detection value reaches a proper level due to an RC time constant of the mean circuit, and even if there is an interfering wave, it is erroneously detected as a free channel.
In other words, where a free channel is searched for according to a received level signal outputted from the mean circuit, an interfering wave also becomes a burst signal, and detection is performed at three points, namely the head, middle and end of the burst. In this case, the time constant of the mean circuit forms a bottleneck in detecting the head and end of the burst.
For example, as shown in FIG. 7, when the interfering wave is partly overlapped with the last end of a used slot (slot to search for a free channel) SO, a received level signal being outputted from the mean circuit is raised by the interfering wave, but at the last level detection point of the used slot SO, the received level signal being outputted from the mean circuit has not been risen to a proper level, so that regardless of the presence of the interfering wave, the level of the received signal at the pertinent level detection point is determined to be lower than the actual level.
In this free channel detection, determining the received signal level to be lower than the actual level causes a problem that the used slot SO is erroneously detected to be a free channel regardless of the presence of the interfering wave.
To remedy such a problem, a time constant of the mean circuit may be lowered, so that the received level signal being outputted from the mean circuit can be risen quickly. But, when a linear modulation method such as QPSK or .pi./4 shift QPSK is used, its envelope is also varied with time, and its received level signal also tends to be affected by the varied envelope.
In other words, when the time constant of the mean circuit is lowered, averaging action is decreased, and variation of the output of the mean circuit increases, resulting in a disadvantage of increasing a detection error.